Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Engine rebuild

I'm rebuilding an early 5 main 1800 for my car. Since this is my 1st engine rebuild, be prepared for a bunch of question over the next little while. So far I've got the whole thing torn apart and am getting ready to take it to a local machine shop. Here's the list of work I think I need them to do:

- hot tank / clean
- check for cracks
- install supplied bronze valve guides
- install hardened valve seats
- cut 3 angle valve seats
- skim block mating surface for flatness

- hot tank / clean
- check for cracks
- bore cylinders to fit .020 oversize pistons
- skim head mating surface for flatness

- cut main journals to fit .020 oversize bearings
- cut rod journals to fit .020 oversize bearings
- balance *

- install supplied ring gear
- resurface clutch face
- balance *

- machine MGA rear plate to accept MGB oil seal

Should I ask them to balance all rotating parts together as a whole rather than (or in addition to) individually balancing the crank and flywheel? Do the new guides need to be reamed to fit the new valves? Do the new valves need to be cut? Have I forgotten anything obvious?

Andy Bounsall

I am nor a big fan of paying for things it may not need. IMHO, 3-angle valve grind is a waste of time and money, unless you have money to burn and think it might net a small increment of power. This is usually reserved for high cost modified engines doing extensive porting for max power.

Have the block bored before you buy pistons, as you don't know how much needs to be bore out for clean up. Many old blocks need 0.030" overbore. Buy pistons to suit after boring. Blocks commonly do not need to be skimmed, so check flatness before you pay to have it done. The most common problem spot is between #2 & #3 bores.

Have the crank checked/ground before buying bearings. Most old cranks do need to be ground once, commonly 0.010 u/s for main journals and 0.020" u/s for rod journals (but could be more or less). Buy bearings to fit after grinding.

If you feel the need to do balancing, then balance con-rods, crankshaft and flywheel separately. If you have a 2000-gram digital postal scale you can easily do con-rod balancing at home. However, consider having the big ends checked for roundness. If they are out of round they can be skimmed and re-bored to restore roundness.

Flywheels are well balanced from factory and usually stay balanced, even after changing a ring gear and light refacing. If you have it machined for lightening, then it needs to be re-balanced. You might consider removing 8-pounds, machining the MK-I flywheel to late MK-II or early MGB spec's. Most people like the end effect.

I have had a crankshaft checked for balance a few times after regrinding, finding it is still in balance as original. If you have even a single bearing journal welded for build up, then all journals need to be reground because it would like warp a little, and it should be checked/balanced afterward.
Barney Gaylord

Be careful about having the rods rebuilt if your thinking about it.Sometimes they clean up the sides of the big end,and depending if it's been done before you might end up with to large of side clearance.
gary starr

Andy, I don't understand your last point about the MGA end plate. This is a B engine.
After hot tanking, you will need new cam bearings installed and maybe line reamed depending on what you buy.
Skimming head - again check it first. Too much skimming could result in too little valve clearance to the head.
The shop will probably want the pistons first. so if you think .020 is enough, be sure, or ask them to look. They may also want the main bearing caps in order to align the boring machine square.
I'm going through the same process, fortunately I only need a rebore. (MGA Mk 2)
Art Pearse

The last time that I rebuilt an engine, I left it to the "reputable" machine shop to purchase the appropriate bearings, etc. They will know what sizes are needed when they complete the machining but not before. But that was over 20 years ago. I did buy the pistons for one build before the machining because I was on a deadline and there was, at the time, a limited supply of oversize pistons. they bored and honed to the size required for the pistons supplied.. I could have had custom pistons made at about the same price as Moss but they required a longer leadtime. I can recommend at least looking at your machinists suppliers. They may be better priced than Moss.

Not mentioned here yet is to remove the engine ID tag. If you really "hot tank" the block, it will dissolve. Also run a hanger wire through all the heater valve passages in the block. These may not clean out in the tank.
C Schaefer

Thanks for the feedback thus far. It's all good.

Art, my car is a '56 with low mount starter. Mating the 5 main to my existing Sierra gearbox requires swapping the MGB rear plate with a low starter MGA rear plate, and it needs to be machined to accomodate the 5 main engine rear oil seal.
Andy Bounsall

Andy, have the machine shop determine how much oversize to rebore the block and turn the crank. Add new cam bearings and line bore. Add new core plugs. I'd recommend that you have the machine shop assemble the short block, which will ensure that the crank, con rods, wrist pins and pistons are installed correctly. Also have them install the camshaft.

Andy 60 Coupe
Andy Preston

This thread was discussed between 04/03/2009 and 05/03/2009

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.